Field of the Invention
The present invention relates to a method for patterning in semiconductor manufacturing. Specifically, it relates to EUV shot noise mitigation methods to be applied in patterning (lithography) that utilizes Photo-Sensitized Chemically-Amplified resist (PS-CAR).
Description of Related Art
In lithographic patterning of semiconductor devices, shrinking technology nodes, and thus feature sizes are driving wavelengths into the extreme ultraviolet (EUV) range. At this time, EUV light sources are still under active development, and currently are not capable of developing and delivering the levels of illumination of prior generations of light sources. To address these shortcomings and still be able to utilize the current generation of EUV light sources, a resist chemistry and associated methods have been developed, called Photo-Sensitized Chemically-Amplified resist (PS-CAR). PS-CAR, like traditional Chemically-Amplified resist (CAR), utilizes acid generated within the resist feature for deprotection, but acid is generated in a two-step illumination process, unlike CAR where only a single patterned exposure is used.
In PS-CAR, a first EUV patterned exposure is used to generate a pattern (latent image within the resist) with a relatively small amount of generated acid, and at the same time generate a photosensitizer (PS) compound, for example from a photosensitizer generator added to the PS-CAR resist. Both the acid and photosensitizer (PS) are generated only in illuminated portions of the PS-CAR resist, during the first EUV patterned exposure. Thereafter, a second flood exposure is performed, i.e. with no pattern, at a second wavelength of light different than the wavelength of the first EUV exposure. The PS-CAR resist chemistry is chosen such that the photosensitizer (PS) is sensitive to the second wavelength of light used in the second flood exposure, while other resist components are not. The photosensitizer (PS), wherever present in the pattern formed during the first EUV patterned exposure causes further generation of acid during the second flood exposure, with tenfold increases of acid concentration, for example, being typical. This photosensitizer-induced acid concentration increase results in greatly increased contrast, which allows more process latitude particularly with respect to the RLS tradeoff (Resolution-Line Width Roughness-Sensitivity). Thus, PS-CAR presents an enabling technology for EUV lithography because it allows the productive use of EUV sources and lithography at their present power levels.
It should be noted here that PS-CAR processes may involve additional steps, for example between the first EUV patterned exposure and the second flood exposure. The above description was simplified for purposes of clarity. Also, in some PS-CAR chemistry embodiments, no acid may be generated during the first EUV patterned exposure, and only photosensitizer (PS) may be generated, which generated photosensitizer (PS) causes generation of all acid during the second flood exposure. Alternatively yet, acid may be generated in small quantities, as explained before, but it may be effectively consumed by competing chemical reactions, such as quenching events (depending on the amount of quencher present in the PS-CAR resist).
Because the power levels of EUV sources are so low, and because EUV photons each have a relatively high energy due to the short EUV light wavelength, there are relatively fewer photons involved in the patterned EUV exposure (for the same exposure dose) than when longer wavelength light sources are used (e.g. ArF, KrF, etc.) This means that there will be fewer acid molecules generated, for example, in a traditional chemically-amplified resist (CAR). As the EUV exposure dose is decreased, the stochastic probability increases of a photoacid generator (PAG) molecule not being decomposed into acid within a given volume, leading to uneven acid concentration within the resist—an effect collectively known as shot noise. Shot noise is known to be one of the major causes of line width roughness (LWR) in patterning. Depending on the feature type, other parameters such as line edge roughness (LER) or contact edge roughness (CER) may also be affected, where applicable.
Shot noise can also manifest itself during the first EUV patterned exposure of the PS-CAR process, both in an uneven acid concentration and in an uneven photosensitizer (PS) concentration. Further amplification of these uneven concentrations of acid and/or photosensitizer (PS) during the subsequent second flood exposure would lead to degradation of process performance, with line width roughness (LWR) being severely affected (or LER or CER depending on the feature type). Therefore, the need exists for methods to mitigate shot noise, specific to PS-CAR lithography and patterning.